The present invention is directed to a device for imaging printing plates using an array of n laser diodes.
For some time now, devices and methods have been known, which make it possible to image a printing plate, whether it be a flat or curved surface, through exposure to laser radiation. Devices and methods of this kind are used, in particular, in so-called CtP systems, computer-to-plate, or direct imaging print units or printing presses for manufacturing offset printing forms.
At the present time, printing plates are primarily imaged by laser diode systems. Their inherent system properties prevent them from reaching the physical limits of the beam quality. In particular, their low beam quality limits their depth of focus, so that an autofocusing system is needed at high resolutions. Two different approaches are currently used for multibeam imaging, i.e., for simultaneously exposing a plurality of image points on various media, such as printing plates, films, data carriers or the like. On the one hand, the radiation from individual laser diodes or an array of laser diodes can be directly applied via optical elements, such as lenses, mirrors or fibers, to the medium to be imaged. On the other hand, the radiation from a laser light source, typically laser diode bars, can be projected via diverse optical elements onto an array of n modulators. For the most part, these are electrooptical or acoustooptic modulators. By selectively driving the n modulators, one can select individual beams from the entire radiation and modulate their power. The selected, power-modulated beams are supplied via further optical elements to the medium to be imaged.
European Patent Application No. 0,878,773 A2 describes an optical system for imaging an array of light sources, in particular an individually addressable array of laser substantially greater than their emitter height. The emission region is typically about 1 micrometer high and 60 micrometers wide. The optical system is composed of a system of non-anamorphotic imaging lenses and of a cylinder lens, which is placed between the array and the imaging lens system and images the laser radiation onto the scanning surface. This surface usually does not lie in the foci of the laser beams, so that a widening of the short dimensions of the imaged emission surface occurs.
U.S. Pat. No. 5,521,748 describes a system for exposing image data using an individual laser or an array of diodes and a light modulator. The light transmitted by the laser or the array is imaged onto a modulator having a row of light-modulating elements of the reflectance or transmittance type. Once selection and power modulation are carried out, the radiation is imaged onto a surface having light-sensitive material, forming individual image points. To place image points of this kind on a complete, two-dimensional surface, a relative motion of the image points to the light-sensitive material is provided. In the interplay resulting from generation of the individual points and the relative motion, the desired image data are then exposed on the two-dimensional surface. The relative motion between the light beams emanating from the light modulator and the light-sensitive material can be effected on a cylindrical configuration such that lines are exposed in a meander shape along the axis of symmetry of the cylinder, or such that lines run around the cylinder in a helical form.
U.S. Pat. No. 5,691,759 discusses a multi-beam laser light source, which produces raster scan lines on a medium using the so-called interleaving raster scan line method. The interleaving raster scan line method is distinguished by the following properties. A laser light source emits radiation, from which n image points are produced using modulated power by employing suitable imaging optics and modulation. These n image points are arranged in a row, and the distance between two adjacent points is (n+1)p, p being the distance between the dots. Provision is made between the medium and the image points for a relative motion in both directions, spanning the surface of the medium. Once n points are imaged, the medium is displaced relatively to the image points with a translational component that is perpendicular to the direction defined by the axis of the image points, so that n points can again be exposed at another location of the medium. In this manner, so-called scan lines of image points are formed, initially at a distance of (n+1)p, which are produced by laser radiation, whose power is modulated in dependence upon the image information. Upon completion of a scan having a translational component in the perpendicular direction, a displacement by the distance (n×p) follows in parallel to the direction defined by the axis of the n image points. The n image points are then shifted again with a translational component that is perpendicular to the direction defined by the axis of the image pixels on the surface, forming further scan lines. Thus, each raster scan line is separated from its immediate neighbor by the pitch distance p between the dots. Using a plurality of optical beams from a laser light source, an overlapping of the scan lines ensues (interleaving raster scan line method).
An enhanced interleaving raster scan line method for a multibeam laser light source is described in European Patent Application No. 0,947,950 A2. In the case of n image points having a pitch distance p of the dots, each of whose adjacent image points are separated by the distance (q×n+1)p, q being a natural number, an incremental distance of n×p results by which the medium must be moved between the marking of two scan lines. An overlapping (interleaving) of the scan lines is thereby achieved, in other words, the new scan lines are written between the old scan lines. By properly selecting the displacement in parallel to the axis defined by the image points, by the distance n×p, an imaging is then possible, without one location, where image information is to be scanned, being repeatedly exposed to one image point of a laser. What distinguishes the described method is that adjacent image points of the laser diodes are spaced further apart, in each case, than the width of the displacement by which the medium is moved between the old and new scan lines.
Various disadvantages are associated with each of the known devices. The radiation emitted by broad array laser diodes, laser diode bars, and laser diode stacks exhibits a low beam quality, as quantified by the diffraction index M2. Even with correction, the attainable depth of focus is only suited for imaging at a low resolution, typically 1,270 dpi. Therefore, to produce very small dots, for example resolutions of about 2,540 dpi, an autofocusing system is necessary, which requires a complex mechanical and electrical design. If the light source and modulator are provided separately, there is an increased requirement for optical, electronic and mechanical components, as well as for substantial overall space. Many components need to be adjusted, and the service life can be clearly limited. The temperature management of the components turns out to be just as problematic. Only a limited, minimal physical size is possible when a device for imaging printing plates is assembled from discrete components. The described interleaving raster scan line method is not suited for compact laser light sources, since the distance between adjacent image points must always be one unit p greater than the number of beams, so that one must revert to scanning methods in which image points are set densely together.